This document discusses isotopic substitution, which is the replacement of atoms in a molecule with isotopes of different mass. Isotopic substitution is useful for vibrational spectroscopy because it changes the reduced mass and normal modes of vibration, leading to different vibrational frequencies. Specifically, substituting heavier isotopes lowers vibrational frequencies due to an increase in reduced mass. Examples given are substituting deuterium for hydrogen in HCl, which lowers frequencies by a factor of 1.35-1.41, and substituting 13C for 12C in CO, which also lowers vibrational energy levels.

1. SUBMITTED TO : SUBMITTED BY :
DR.AKHIL SAXENA APURVA
19MSC1144
SECTION :1

2. CONTENTS
1. INTRODUCTION
2. BASIC NEED OF ISOTOPIC SUBSITUTION
3. EFFECT OF ISOTOPIC SUBSITUTION
4. APPLICATION OF ISOTOPIC SUBSITUTION
5. EXAMPLES OF ISOTOPIC SUBSITUTION
6. REFRENCES

3. INTODUCTION
 WHAT IS AN ISOTOPE
Isotopes are the atoms that share the same number
of protons but differ in the number of neutrons
contained in the nucleus, thus giving these atoms
different mass numbers

4. BASIC NEED OF ISOTOPIC
SUBSITUTION
 Isotopic substitution is a useful technique due to the fact that
the normal modes of an isotopically substituted molecule are
different than the normal modes of an unsubstituted molecule,
leading to different corresponding vibrational frequencies for
the substituted atoms.
 Vibrational spectroscopy is done in the infrared region of the
electromagnetic spectrum, which ranges from around 10-
6 to 10-3 meters. IR and Raman spectroscopy observe the
vibrations of molecules, displaying the normal and local
modes of the molecule in the spectra.
 The specific mass of each atom will affect the reduced mass
of the overall molecule, therefore changing the vibrational
frequencies of that molecule.

5. EFFECT OF ISOTOPIC SUBSITUTION
 When an atom is replaced by an isotope of larger mass, µ
increases, leading to a smaller νeνe and a downshift (smaller
wavenumber) in the spectrum of the molecule.
 And the reason is explained below :
Molecular vibrations are often thought of as masses attached by
a spring
(shown in figure)

6. and Hook’s law can be applied
F=−kx
where,
 F is the resulting force,
 x is the displacement of the mass from equilibrium and
 k is the force constant, defined as
in which V(r)=12k(r−req)V(r)=12k(r−req), which
comes from incorporating Hook’s law to the harmonic oscillator. The diatomic
molecule is thought of as two masses (m1 and m2) on a spring, they will have a
reduced mass, µ, so their vibrations can be mathematically analyzed.
μ=m1m2/m1+m2
 When an atom in a molecule is changed to an isotope, the mass number will be
changed, so µµ will be affected, but k will not (mostly). This change in reduced
mass will affect the vibrational modes of the molecule, which will affect the
vibrational spectrum.
 Vibrational energy levels, νe, are affected by both k and µ, and is given b

7. APPLICATION OF ISOTOPIC EFFECT
 Substituting atoms with isotopes has been shown to be very
useful in determining normal mode vibrations of organic
molecules. When analyzing the spectrum of a molecule,
isotopic substitution can help determine the vibrational modes
specific atoms contribute to. Those normal modes can be
assigned to the peaks observed in the spectrum of the
molecule.
 Isotopic substitution can also be used to determine the force
constants of the molecule. Calculations can be done using the
frequencies of the normal modes in determining these values,
based on both calculated frequencies and experimental
frequencies.

8. EXAMPLES OF ISOTOPIC EFFECT
 Taking the diatomic molecule HCl, if the hydrogen is replaced
by its isotope deuterium, µ is doubled and therefore νe will be
decreased by 2–√2. Deuterium substitution leads to an
isotopic ratio of 1.35-1.41 for the frequencies corresponding to
the hydrogen/deuterium vibrations. There will also be a
decrease by 2–√2 in the band width and integrated band width
for the vibrational spectra of the substituted molecule. Isotopic
substitution will affect the entire molecule (to a certain extent),
so it is not only the vibrational modes for the substituted atom
that will change, but rather the vibrational modes of all the
atoms of the molecule. The change in frequency for the atoms
not directly invovled in the substitution will not display as large
a change, but a downshift can still occur.

10.  On changing from 12C16O to13C16O, atomic mass
increases, B decreases, so the energy of levels becomes
lower.

11. REFRENCES
 https://chem.libretexts.org/Bookshelves/Physical_and_Theore
tical_Chemistry_Textbook_Maps/Supplemental_Modules_(Ph
ysical_and_Theoretical_Chemistry)/Spectroscopy/Vibrational_
Spectroscopy/Vibrational_Modes/Isotope_effects_in_Vibration
al_Spectroscopy#:~:text=Isotopic%20substitution%20is%20a
%20useful,frequencies%20for%20the%20substituted%20ato
ms.
 http://www.physics.dcu.ie/~be/Ps415/Rotational1.pdf